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<center><h3>Solids in Chemical Equilibria</h3></center>

<p>Solids have names ending with either:
<ul>
<li>&#147;(cr)&#148; or &#147;(c)&#148; for crystalline;</li>
<li>&#147;(am)&#148; for amorphous; or</li>
<li>&#147;(s)&#148; for solid (unknown crystallinity).</li>
</ul>
</p>

<p>For solids, like for aqueous complexes, the <nobr>log<i>K</i>-value</nobr> for
the formation reaction is used. Note that this is the inverse of the
<i><u>solubility product</u></i>, i.e.:  <nobr>log<i>K</i><sub>f</sub></nobr> =
<nobr>&#8722;log<i>K</i><sub>sp</sub></nobr></p>

<p>Owing to the Gibbs phase rule, the number of solids that may be present
simultaneously in a chemical system is limited.</p>

<p>Unlike aqueous species, which have concentrations that may become gradually
small, solid phases either are stable, or not. The concentration usually changes
in a narrow range of conditions between the maximum
possible amount, and zero.</p>

<p>If a solid is present in a system, its composition is constant, and its
activity may be set equal to one <nobr>(log <i>activity</i></nobr> = <nobr>0).</nobr>
Its concentration in mol/(L of solution) may be calculated and plotted.
If the solid is not present, its concentration is zero, and its activity is undefined.
In that case the programs calculate the <i><u>saturation index</u></i>
<nobr>(<i>SI</i>)</nobr> instead of the activity:</p>

<center><nobr><i>SI</i></nobr> = <nobr>log<i>K</i><sub>f</sub>&deg;</nobr>
        <nobr>&#8722;&nbsp;log&nbsp;<i>IAP</i></nobr></center>

<p style="margin-top:0ex"><a name="IAP"></a>
where <nobr><i>K</i><sub>f</sub></nobr> is the equilibrium constant for the
formation of the solid, and <nobr><i>IAP</i></nobr> is the
<i><u>ion activity product</u></i> for the solid (the actual product of
activities as defined in the equilibrium constant of formation).</p>

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